1. Field of the Invention
The present invention relates to a method and apparatus for identifying valid data cells in a redundant path combining unit of an asynchronous transfer mode switch having duplicated switching planes.
2. BACKGROUND INFORMATION
In a known type of validity checking apparatus, the redundant path combining unit is arranged to look at a sequence number attached to each data cell, to detect whether the sequence number matches that stored in a RAM or is greater by one. If either of these conditions are satisfied the data cell is considered valid and the sequence number is incremented.
A disadvantage of this form of validity checking is that if the same two consecutive data cells are lost from both planes, the redundant path combining unit would not allow any data cells through until the sequence count had cycled round. For a 64 data cell sequence follows: EQU Plane 0 data 1, R, B, 4 . . . 63, 0, ,1, 2 EQU Plane 1 data 1, R, B, 4 . . . 63, 0, ,1, 2
If data cells 2 and 3, for example, are lost in both planes, then the following data cells 4, 5, 6 . . . 63, 0, 1 are rejected in the cycle resulting in a total loss of 64 data cells. This results in the needless prolongation of the original data cell-loss error condition.